Transformers are used for power transmission for power supply purposes, by adapting the voltage from a first voltage level to a second. Power transformers of dry design, so-called dry transformers, are being increasingly used instead of power transformers with an oil filling, as were previously widely used.
In this case, the configuration of a power transformer of dry design is very similar to that of a power transformer with an oil filling to the extent that the respective winding formers are also fitted to cores composed of ferromagnetic material in a power transformer of dry design, which cores are each connected to yokes at both ends, and form a magnetic circuit.
However, in the case of dry transformers, the heat losses which were absorbed by the oil in the case of power transformers with an oil filling and were emitted via suitable cooling surfaces or separate coolers, are dissipated by air convection. With the lower specific heat capacity of the air in comparison to oil, the power of dry transformers can be restricted.
Resistive losses occur in the windings of a loaded transformer because of the winding currents and eddy currents in the conductor material. These resistive losses have no-load losses superimposed on them, and possibly short-circuit losses as well as hysteresis losses.
The no-load losses are governed primarily by the induction and the nature of the core and are approximately independent of the operating temperature of the transformer. The short-circuit losses are temperature-dependent and rise with the temperature and the specific resistivity of the conductor material, if the load is constant. Core materials having a very low hysteresis loop can therefore be used in order to keep the hysteresis losses as low as possible.
In order to reduce the heat losses caused in this way in a dry transformer, and thus to improve its load capability, amorphous core material has recently been used, rather than grain-oriented core material.
However, the use of amorphous materials can involve new designs and processing forms since the amorphous material is highly pressure-sensitive, as a result of which this can result in an increase in the core losses.